(1) Field of the Invention
The present invention relates generally to broadband antennas and, more particularly, to a direct fed, rugged bifilar helix antennas.
(2) Description of the Prior Art
Broadband helical antennas utilized for satellite communications bands may be mounted on the mast of a surface vessel for wideband satellite communications. Satellite communications may include Demand Assigned Multiple Access (DAMA) UHF satellite communications.
U.S. Pat. No. 6,246,379, to the present inventor, which is briefly discussed hereinafter, provides a quadrifilar antenna suitable for broadband satellite communications that is of moderate size, moderate weight, rugged, and does not require matching networks. Above a cut-in frequency, antennas of the type described in U.S. Pat. No. 6,246,379 have a broadband, approximately constant resistive impedance equal to approximately the characteristic impedance (Z0) value of the antenna, resulting in a low voltage standing wave ratio (VSWR) about the antenna Z0. By making the antenna elements as wide as practically possible before they overlap, the application antenna reduces the value of Z0 to a practical lowest limit of 100 ohms, which feeds very well into the Z0 of 100 ohms between the two center conductors of a 180 degree power splitter feeding a given bifilar. Thus the resultant antenna of two crossed bifilar helixes has a 50 ohm 90 degree power splitter feeding two 50 ohm 180 degree power splitters feeding their two 100 ohm outputs directly into the two crossed bifilar helixes making up the quadrifilar helix. There are no matching networks. The antenna is directly fed via its power splitter feed network.
Detailing the construction of the antenna, U.S. Pat. No. 6,246,379 discloses a quadrifilar helix antenna that includes a base portion for containing a feed network including a power input, a 90 degree power splitter in communication with the power inlet, and first and second 180 degree power splitters in communication with the 90 degree power splitter. A support tube is mounted on the base portion, and a plurality of disk separators are mounted on the tube. Four elongated filar elements are wound around the tube and are spaced therefrom by the disk separators. The elements are connected to end-most lower and upper ones of the disk separators; the elements extending toward a center feed point of the upper disk separator. First and second radially opposite pairs of feed cables are wound around and connected to the centers of the elements, extending from the lower disk separator to the upper disk separator, to function as an infinite balun. At the lower disk separator, the cables are introduced onto the antenna at the lower ends of the elements where both the ends and cables are shorted together. At the upper disk separator, the cables are opened up to feed the upper ends of the elements. A given radially opposite pair of cables feed the radially opposite elements of a given bifilar element pair. Thus a given 180 degree power splitter feeds a given bifilar pair of elements.
In a co-pending patent application entitled DIRECT FED BIFILAR HELIX ANTENNA (Navy Case No. 83514), by Michael J. Josypenko, the same named inventor to this application, there is a description of how the broadband impedance properties of quadrifilar helixes can also be applied to bifilar helix antennas, since the quadrifilar helix is an array of two crossed bifilar helixes. As derived in the application, the difference in the impedance from the quadrifilar design of two crossed bifilars to the single bifilar is that when changing from two crossed bifilars to a single bifilar, with the width of a bifilar element being the combined widths of the two quadrifilar elements it replaces, Z0 is halved to approximately 50 ohms. The result is a bifilar helix that is fed directly from a 50 ohm coaxial line that uses the antenna as an infinite balun to reach the feed point of the antenna. For an antenna of 50 ohms, the constant width of the antenna elements are at a practical maximum of the space available for an element. A difference between the quadrifilar helix and bifilar helix antenna is that the bifilar antenna must always be fed in back fire mode and must be long enough to be a traveling wave antenna before unidirectional patterns of cardioid shape occur off of the fed end of the antenna, and therefore it may be required to be longer than the quadrifilar helix antenna.
U.S. Pat. No. 6,288,686, issued Sep. 11, 2001, to the present inventor, M. Josypenko, discloses a tapered direct fed quadrifilar helical antenna having a feed point for the antenna connecting to individual helical antenna elements. Each antenna element tapers from a maximum width at the feed point to a minimum width. The tapered antenna elements provide impedance transformation. The antenna produces a cardioid pattern that corresponds to antennas having constant width antenna elements. The elements of the tapered direct fed quadrifilar are made narrower and lighter than an untapered quadrifilar by applying the principle of matching two impedances with a half wavelength tapered transmission line. A given two elements of a bifilar of the antenna are tapered to become a radiating tapered transmission line, matching the input impedance of 100 ohms at the feed point where the elements are of maximum width to a higher impedance at least one half wavelength down the elements where the elements have been tapered down to be much narrower. The advantage of narrower elements is reduced weight of and amount of material required for the antenna.
Accordingly, the above cited prior art does not disclose a less complex antenna that occupies a small diameter (e.g., 0.1 to 0.3 wavelengths) and that avoids the need for power splitters or matching networks, requires only one feed cable, and provides a simplified design with only two antenna elements.